We use the HAMZA ( ء ) above the alef ( ا ) when we can really pronounce it like in : أ)حمد . (أ)خضر. (أ)صفر) We use the alef without hamza when we pronounce it lazily not hard for ex: ا)ستعمل .(ا)عتصم . (ا)ستفاد)

it's a specific grammatical question ! Just in the cases below the HAMZA symbol shouldn't be written, and we use (ا) (همزة الوصل): 1- in names: ال التعريف (al-) اسم ، ابن ، ابنة ، ابنم ، امرؤ ، امرأة ، اثنان ، اثنتان ، ايم الله ، ايمن الله ، است 2- in verbs: imperative (فعل الأمر) from threefold verbs (ارسم، العب..) imperative and past and gerund from fivefold and sixfold verbs (انتصِرْ، انتصَرَ، انتصار) source: http://www.drmosad.com/index116.htm and you can watch this lesson: https://www.youtube.com/watch?v=cIjQ1l-xudM

This is based on the answer of an old argument about the number of Arabic letters, some linguistics say they are only 28 and hamza is merely another form for alif, others see that they are 29 and Arabs neglect writing ء over ا just like they do with those tiny vowels over letters - FATHA-DHAMA-KASRA-SKOON. the easiest way to solve this in Arabic and all languages is to deal with words as if they are individuals - I know this word because I have met it before - as for new ones they must be introduced for me.